The present invention relates to a lateral bipolar transistor, in particular on a SOI substrate.
Bipolar transistors are an essential component in integrated circuits on an SOI substrate which are produced in the context of a biCMOS process. A fundamental problem with lateral bipolar transistors is the relatively high terminal resistance at the base. Given a lateral arrangement of the emitter, base and collector, the base is a narrow strip between the emitter and the collector. It is usually necessary to make contact with this narrow strip at the end, that is to say laterally with respect to the longitudinal direction of the transistor, by means of a heavily doped terminal region. With this structure, the base current flows through the relatively high-resistance base. The sheet resistance of the base is limited by the boundary conditions resulting from the required current gain. It is not possible to achieve a sheet resistance better than about 10 to 20 kohm/sq. A direct low-resistance base terminal, for example by means of polysilicon applied to the base region, fails because of the rapid diffusion of the dopant out of this polysilicon into the semiconductor material of the base, as a result of which the charge carriers in the base region would be influenced in an uncontrollable manner. The publication by T. Hori et al., in IEEE Trans. Electron Devices 36, 340-350 (1989) and the literature cited therein investigate the electrical and physical properties of nitrided oxide layers. The publication by T. Morimoto et al. in IEDM 90, 429 -432 (1990) describes the properties of nitrided oxide layers on silicon as a diffusion barrier for boron.
The object of the present invention is to specify a lateral bipolar transistor which is suitable in particular for production in the context of a biCMOS process on an SOI substrate and has a low-resistance base supply, and a production method therefore.
In general terms the present invention is a lateral bipolar transistor having an emitter region, a base region and a collector region in a semiconductor layer and having a base terminal region. There is present, on the base region, a base electrode which is made of doped polysilicon material and which is separated from the base region by a diffusion barrier. The diffusion barrier is thin enough to enable charge carriers to pass through the diffusion barrier to a degree which suffices for the functioning of the transistor. The base electrode is electrically conductively connected to said base terminal region.
Advantageous developments of the present invention are as follows.
The semiconductor layer is mohocrystalline silicon. The semiconductor layer is a body silicon layer of an SOI substrate.
The diffusion barrier is nitride.
The present invention is also a method for producing a bipolar transistor having the following steps:
a) a region which is provided for the bipolar transistor is provided with a doping envisaged for the base region and is covered with an oxide layer;
b) a nitride layer is produced in a nitrogen-containing atmosphere at the interface between said oxide layer and the semiconductor layer;
c) using a mask, a residue of the oxide layer is removed in a region provided for the base electrode;
d) a polysilicon layer provided for the base electrode is applied and, is structured such that a portion which is provided for the base terminal region and a portion which is provided for the base electrode, which portions are connected to one another, remain and that the region provided for the base region is covered by polysilicon;
e) using masks, implantations of dopants are made for the emitter region and the collector region and also for the base terminal region;
f) the dopants which have been introduced are activated and diffusion of the dopant which has been introduced into the polysilicon out of the base terminal region into the base electrode is effected.
In a development of the method of the present invention, in step a) the oxide layer is applied to a thickness of at most 10 nm, and in step b) the nitride layer is produced with a thickness of at most 2 nm.
In the transistor according to the invention, a low-resistance base supply is realized by a base electrode which is made of polysilicon and is applied to the base region. The polysilicon is heavily doped for low resistance. A diffusion barrier situated between the actual base region and this base electrode prevents diffusion of dopant from the electrode into the semiconductor material of the base and simultaneously ensures sufficiently good electrical contact between the base electrode and the base region of the transistor. This diffusion barrier consists of silicon nitride, for example, which constitutes an efficient barrier against dopant diffusion, yet is thin enough for there to be electrical contact between the base electrode and the base region.